Rapid microwave-assisted flux growth of pure β-NaYF₄: Yb³⁺, Ln³⁺ (Ln=Er, Tm, Ho) microrods with multicolor upconversion luminescence

Pure hexagonal β-NaYF₄:Yb³⁺, Ln³⁺ (Ln=Er, Tm, Ho) microrods were successfully synthesized for the first time through a rapid microwave-assisted flux cooling method. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectra were used to characterize the samples. The results indicate that with the prolonged time, the phase of product transforms from orthorhombic YF₃ to hexagonal NaYF₄, while the morphology changes from nanoparticles to hollow microtubes then to solid microrods. The microwave (MW) heating technique allows for a reduction of about 90% in preparation time, making it possible to obtain pure β-NaYF₄ microrods within only 12min. By changing the dopant's species, multicolor (yellow, blue, and green) upconversion (UC) emissions can be obtained in β-NaYF₄:Yb³⁺, Ln³⁺ under 980nm laser diode (LD) excitation. The UC mechanisms in Yb³⁺/Ln³⁺ co-doped β-NaYF₄ samples were analyzed in detail based on the emission spectra and the plot of luminescence intensity to pump power. Importantly, this synthetic methodology may offer a new alternative in the preparation of high quality rare earth fluorides.